Tom Lane, 28.08.2012 16:30:
>> In Oracle an index (entry) has the information about transactional visibility.
>
> You sure about that?
Yes, although technically it's not the index *entry*, but the index *block*.
But the result is the same thing.
The visibility information is stored on data block level. And an index block is not really different to a table block
whenit comes to transaction handling.
> What I always understood about Oracle is that the main table (and by
> implication, also the indexes) has *only* the most up-to-date version
> of any row.
Yes that is true. The block stores a pointer to the "old" versions (through the so called SCN - comparable somehow to
PG'stxid).
Depending on the transaction reading the block it will either follow that link (if it's a different transaction) or use
thatblock (if it's the transaction that modified the block in the first place).
This is somewhat simplified, but I think it's a good enough picture.
If you are interested in more details, see the link to the concepts manual I posted in the response to Craig
Thomas
